The invention refers to two bearing arrangements including a magnetic radial bearing and a touchdown bearing for contactless support of a rotor shaft and for catching said rotor shaft of a turbomachine with a power output of 1000 kW and more, wherein both bearings, in axial alignment, are accommodated in a common bearing housing.
Furthermore, the invention refers to two bearing blocks for a turbomachine with a power output of 1000 kW and more, wherein the respective bearing block is arranged axially on the outside with regard to the turbomachine and has a magnetic radial bearing, accommodated in a common bearing housing, for contactless support of a rotor shaft, and a touchdown bearing for catching said rotor shaft of the turbomachine, and wherein the two bearings, in axial alignment, are accommodated in the bearing housing.
Such a previously described bearing block is known from Japanese laid-open specification JP 6335199 A.
A bearing block with a bearing having a practically zero longitudinal play is described in German laid-open specification DE 30 11 078 A. The bearing block there has a bearing with rolling constructional elements. At least one race of the bearing is fixedly connected to a magnetic device and this magnetic device is arranged so that it provides a relative spacing relationship in the axial direction of the bearing block for the races of the bearing in order to practically eliminate the longitudinal play of the bearing.
A turbomolecular pump for use in sealed vacuum systems is known from the translation of DE 696 25 870 12 of European patent EP 0 768 467 B1. The bearing unit which is disclosed there comprises a stator-side, passive, radial magnetic bearing with a multiplicity of permanent magnets. Further permanent magnets, which are arranged on a rotor of the turbomolecular pump, lie opposite these permanent magnets, wherein in the controlled state like magnetic poles face each other in each case. The bearing unit, moreover, comprises a safety bearing element (emergency safety bearing element) which is fixedly connected to the stator-side part of the magnetic bearing in order to prevent direct contact between the rotor and the stator. Furthermore, the stator-side part of the magnetic bearing is movably suspended in the stator and pretensioned in the axial direction by means of a cylindrical, elastic pretensioning means in order to be able to interact with an active magnetic thrust bearing of the bearing unit for axial position control of the rotor. Furthermore, as a result of the pretensioning means the resonant vibrations in the radial direction of the main shaft or of the rotor, which arise during run-up of the rotor, are absorbed.
In the case of the rotating machines, they are preferably turbomachines, such as turbogenerators, turbomotors or turbocompressors. The maximum operating speed of such machines customarily lies at more than 4000 min−1. They typically have a power output of 1000 kW and more. In the case of a turbocompressor, an electric motor unit drives a turbine unit. The rotor shaft of the electric motor unit and the turbine shaft are preferably arranged in alignment. Both shafts can be interconnected via a coupling element.
In the case of the known machines, active magnetic radial bearings for supporting the rotor shaft are used to an increasing extent instead of plain bearings in order to reduce friction losses there. In this case, the load-bearing capacity is created by controlled electromagnets. In the case of control failure, the rotor shaft or one rotor-shaft end drops into a touchdown bearing which for a limited time provides the emergency running characteristics of the rotating machines. The magnetic radial bearing and the touchdown bearing can be arranged in a bearing bracket or in a casing of the rotating machine. Alternatively, the magnetic radial bearing and the touchdown bearing can be axially adjacently accommodated in a common bearing housing of a bearing block. Provision is typically made for two bearing blocks for supporting a respective axial shaft end of the rotating machine. In addition, a magnetic thrust bearing, for axial fixing of the supported rotor shaft, can be accommodated in the bearing housing.
The magnetic bearings ensure a contactless, wear-free and stable supporting of the rotor shaft at very high speeds. During operational use, an air gap in the range of approximately 0.5 to 1.0 mm is typically maintained between the radial magnetic bearing and the rotor shaft which is to be supported. Since for system-related reasons a magnetic bearing can fail, provision is made for a touchdown bearing which can support the rotor shaft in the case of failure of the magnetic bearing or in the case of shutting down of the electrical system in general. For this purpose, the touchdown bearing has a slightly larger inside diameter in comparison to the shaft diameter so that the rotor shaft does not make contact with the touchdown bearing when the magnetic support is operational. The air gap between the touchdown bearing and the rotor shaft is somewhat smaller in comparison to the operational air gap in the case of the magnetic radial bearing. The air gap lies typically within a range of 0.1 to 0.5 mm.
A magnetic radial bearing typically has a hollow-cylindrical, annular construction with an annular magnetic core, on the radial inner side of which current coils are introduced for forming electromagnets for the magnetic support. Such magnetic radial bearings as a rule are fixedly connected to a bearing housing or to the bearing bracket by means of a threaded connection. Alternatively, the magnetic radial bearing can be part of a bearing block which is fixedly connected to the bedplate of the rotating machine which is to be supported. All supporting components of the rotating machine, including the magnetic radial bearing, are customarily of a rigid construction.
If the rotor shaft of the rotating machine drops into the touchdown bearing in the case of failure of the magnetic radial bearing, then severe shock loads occur on the entire bearing. These can principally be avoided by means of elastic supporting of the touchdown bearing. However, the available deflection travel of the touchdown bearing is restricted by the air gap of the active part of the magnetic bearing. This must not be exceeded in the case of collapse of the rotor shaft into the touchdown bearing in order to avoid damage and failure of the magnetic radial bearing. The adjustment parameters are therefore severely limited for elastic supporting of the touchdown bearing.
The magnetic radial bearings are connected to the bedplate of the rotating machine or to the bearing bracket or to the machine casing of the rotating machine at least indirectly in a fixed manner, that is to say rigidly or inflexibly. As a result of a rigid connection, the magnetic bearing is particularly sensitive especially at very high speeds. The cause lies in the fact that the position sensors which are required for controlling the magnetic radial bearing are also excited by mechanical excitations from outside, such as by the machine casing or via the bedplate. Consequently, the position sensors detect relative movements between the fixed magnetic bearing and the machine axis, that is to say the (structural) rotational axis of the rotating machine. The relative movements are consequently caused not only as a result of movement of the rotor shaft but also as a result of sensor movement. In such a case, the magnetic bearing disadvantageously cannot be operated in high speed ranges.